Thin films are routinely used in optics for coating, silvering, as filters and the like and are produced, inter alia, with commercial apparatus by reactive vapor deposition of metals, inter alia, in a vacuum with oxygen or other reactive partners.
Thin films are especially overcoatings of dielectric substances or metals having the thickness of a few molecular layers up to a thickness in the order of magnitude of wavelengths of visible and infrared light.
Japanese patent publication JP 1,225,315 discloses a method for depositing pure gallium on a substrate. Thin films of gallium nitride and gallium arsenide are known but technical applications of gallium oxide are not known.
The reference text "Handbook of Chemistry and Physics", 71st edition (1990 to 1991), CRC PRESS, Boca Raton USA, page 4-65 lists the refractive index of crystalline Ga.sub.2 O.sub.3 as n=1.92 to 1.95 and for hydrogenated Ga.sub.2 O.sub.3. H.sub.2 O as n=1.84. The refractive index of other oxides of gallium is not presented.
The article of Hariu et al published in the Japanese Journal of Applied Physics, Volume 16 (1977), pages 841 and 842, discloses a gallium oxide thin film made by reactive vapor deposition. Gallium is thermally evaporated in an 8.times.10.sup.-2 Torr oxygen atmosphere. Tempering is performed at 1,000.degree. C. The refractive index is only given for the amorphous as-deposited film (not tempered) to be typically 1.6. Accordingly, no suggestion is provided that a thin film of the outstanding antireflective properties (caused by the index of refraction of 1.2) could be obtained.